1. Field of the Invention
The present invention is related to apparatus for generating plasma, and more particularly to inductively coupled electromagnetic plasma accelerators.
2. Discussion of Related Art
Directed streams of plasma are used in semiconductor fabrication for etching and for thin film deposition. For example, plasma processing equipment is used to manufacture microelectronic logic circuits and display substrates, e.g., liquid crystal display (LCD) panels. Inductively coupled plasma (ICP) accelerators are a type of plasma equipment widely used in semiconductor manufacturing processes. ICP equipment is favored for its ability to generate plasma streams having relatively high plasma density and good uniformity characteristics. As industry is able to produce smaller semiconductor gate widths, more microelectronic circuitry can be included within a single semiconductor device. Increasingly sophisticated plasma equipment is needed to produce the smaller, faster semiconductor circuitry while keeping the manufacturing yields at acceptable rates.
FIG. 1 depicts a cut-away top perspective view of a plasma accelerator 100. The accelerator 100 has a circular channel 110 bounded by chamber walls 112, 114 and 116 on the inside, outside and top, respectively. The chamber walls 112, 114 and 116 typically consist of a dielectric material. The inside wall 112 and outside wall 114 are generally oriented equidistance apart, bounded by chamber wall 116 and one end and open at the other end to form the chamber 110. One or more internal circular coils 118 are provided on the external portion of inside walls 112, and a number of external circular coils 120 are provided on the external portion of outside chamber wall 114. The accelerator 100 may be configured with a circular anode 122 disposed on the inside top portion of chamber 110. Conventional ICP accelerators with an interior anode often use coil driving frequencies at around 13.5 MHz. The use of this frequency has been found to be acceptable in conventional accelerators for the purposes of plasma generation and heating as well as initially accelerating the plasma. A cathode (not shown) may be oriented outside the bottom, open end of circular channel 110. A supply line 124 feeds gas through the top wall 116 of circular channel 110 to the anode 122 which ionizes the gas. FIG. 2 depicts a cross-sectional view of the plasma accelerator 100 shown in FIG. 1. A number of different gases may be used for a deposition or etching plasmas, including, for example, Ar, F2, Cl2, CH4, GeH4, CF4, SiHn+, either alone, combined with each other, or in combination with O2, H2 and N2.
Another conventional type of plasma accelerator is the traveling wave accelerator. These devices operate by producing a series of magnetic field local maximums moving in axial direction. A traveling wave is attained by using a series of side coils in which the current amplitude and phase can be adjusted and varied in each coil. The local maximums of the Lorentz force FL and the axial electrostatic ambipolar field EZ also move in the axial direction, producing additional plasma acceleration in case of a proper choice of the traveling wave velocity. Note that plasma engines of this type are analogous to alternating-current linear-induction motors. Such plasma motors differ essentially only in the production of the traveling wave (or stator), to which the plasma (or rotor) is coupled.